Hi I have a car speaker hooked up to an amp to my iPod. It doesn't get very good high quality sound so I wanted to add a tweeter with a passive hpf. I'm not quite sure what a good cut off frequency for music would be if any one know I'd be thankful.

Add 3,3 uF nonpollar capacitor in series with the high frequency speaker(tweeter) and then connect this module in parallel to the main speaker.If mids are too loud - decrease the capacitor to 2,2 uF or even to 1uF.

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For music -- if you care about quality whatsoever -- you will most likely need to find out two things: First, what's the recommended cutoff frequency for the tweeter? You'll damage it if you go too far below that point with too much power. A usually safe value is 2500Hz. Then, you want to find out how high the woofer can play before it starts rolling off (or sounds bad). You may need a tone generator to play sweeps from low to high, and decrease the high frequency until it sounds pretty much level throughout its range.

Hopefully, there's some overlap between those two frequencies. This ensures the two speakers will meet in the middle. But, you don't actually want them to overlap much when you're listening to them; they should hand off at some point and slowly fade out higher (woofer) or lower (tweeter) than the crossover point. The best way to accomplish this is to put a cap in series with the tweeter (a highpass filter), and an inductor in series with the woofer (lowpass). It's even better to go "2nd order" and use an inductor in paralllel with the tweeter, and cap parallel with the woofer, so they fade away more quickly out of the desired range.

Look for 2-way 2nd-order crossover calculators for the exact values. You need to know the nominal impedance of both speakers to accurately calculate everything.

Of course, if you just want to make sound, you can do without. But... eggcch...